Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-10-19
2003-09-16
Niebling, John F. (Department: 2812)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S122000, C349S106000, C438S149000
Reexamination Certificate
active
06621544
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an active matrix-type liquid crystal display apparatus which drives liquid crystals by using thin film transistors (hereinafter, abbreviated as “TFT”) and a projection-type liquid crystal display apparatus.
BACKGROUND OF THE INVENTION
Recently, active matrix-type liquid crystal display apparatuses using TFTs as a switching device which drives liquid crystals have been actively developed.
A conventional active matrix-type liquid crystal display apparatus comprises a TFT substrate and a common substrate which are arranged at a predetermined gap so as to face each other, and a liquid crystal layer held within both substrates.
On the TFT substrate, a TFT as a switching device and a display picture element electrode connected with the TFT are formed at a cross point of the scanning lines and the signaling lines arranged like a grid. The scanning line, the signaling line and the picture element electrode are electrically connected to a gate electrode, a source electrode and a drain electrode of the TFT, respectively. In the active matrix-type liquid crystal display apparatus having such a constitution, when a picture signal is applied through the signaling line in a state where a selection signal is applied to the gate electrode of the TFT, a predetermined signal charge is written to a corresponding picture electrode.
Common electrodes are arranged on the whole surface of the common substrate. An appropriate voltage is applied to the common electrodes through a common terminal disposed around the TFT substrate.
A picture element capacitor is formed between the picture element electrode and the electrode facing to the picture element electrode via liquid crystals. The TFT serves as a switching device which controls the charge flow into and from this capacitor.
The constitution mentioned above becomes a circuit element like a condenser, in which a liquid crystal material acts as a dielectric. The liquid crystals are raised up by applying a voltage to the picture element electrode to change transmittance so that picture displaying is carried out.
FIG. 5
(
a
) shows a layout of one picture element electrode of a conventional liquid crystal display apparatus.
FIG. 5
(
b
) shows a cross sectional view of the apparatus along the B-B′ line in
FIG. 5
(
a
). A conventional manufacturing flow will be explained below using
FIGS. 5
(
a
) and (
b
).
First, as an active layer, on an insulating substrate
11
is formed a polycrystalline silicon thin film
12
at a thickness of 40 nm~80 nm. Subsequently, a gate insulating film
13
is formed thereon at a thickness of 80 nm~150 nm by sputtering or a CVD method.
Next, a gate electrode
14
is patterned into a predetermined shape using a metal or polycrystalline silicon having a low resistance. Subsequently, in order to determine a conductivity type of this TFT, phosphate ions are injected from above the gate electrode
14
at a concentration of 1×10
15
(cm
−2
) to form a channel region (polysilicon channel
12
a
) on a polycrystalline silicon thin film
12
underlying the gate electrode
14
.
Then, after a first interlayer insulating film
15
is formed on the whole surface by using a silicon oxide film (SiO
2
film), a contact hole is formed.
After that, a signaling line
16
and a drain electrode
17
are formed by using a metal having a low resistance, such as Al.
Next, a second interlayer insulating film
18
is formed by stacking a silicon nitride film (SiN
x
) and a SiO
2
film. Subsequently, dangling bonds existing in the polysilicon channel
12
a
are hydrogen termination-treated with hydrogen atoms supplied from the SiN
x
film by sintering to improve the transistor performances.
After formation of contact holes, a picture element electrode
19
is formed by using a transparent conductive film such as an indium tin oxide (ITO).
Recently, a liquid crystal display apparatus with a small picture element pitch has been developed, there are an apparatus having a pitch of one picture element of 30 &mgr;m or smaller, and sometimes, of 20 &mgr;m or smaller. Particularly, for a portable projector, a high-resolution liquid crystal display having a diagonal size of 1 inch or smaller is used for downsizing the apparatus as a whole.
In the conventional liquid crystal display apparatus, when the apparatus is made to have high resolution, since the region of the picture element electrode is depressed relative to the wiring around them and the region of the TFT, the picture element electrodes can not be rubbed sufficiently in the alignment of liquid crystals to reduce the orientation force of the liquid crystals on the picture element electrodes, which, in turn, results in the occurrence of display troubles.
Against the above problem, techniques for improving the display performances by planarizing the region of the picture elements are disclosed in JP-A 4-3248120 and JP-A 4-338718.
FIG. 6
shows the constitution of the liquid crystal display apparatus disclosed in JP-A 4-324820 relating to the planarization, and
FIG. 7
shows the constitution of the liquid crystal display apparatus disclosed in JP-A 4-338718.
The liquid crystal display apparatus disclosed in JP-A 4-324820 comprises, as shown in
FIG. 6
, a wiring
21
, a picture element electrode
22
, and an insulating substrate and lower layer
23
. In
FIG. 6
, after forming a layer other than the wiring
21
and the picture element electrode
22
and pattern
23
on the insulating substrate, the insulating film
24
is formed so as to be disposed below the picture element electrode
22
and, then, the picture element electrode
22
and the wiring
21
are formed. When the liquid crystal display apparatus is a transmission type, since a transparent material must be used for the insulating film
24
, for example, a photosensitive transparent polyimide resin is used.
In JP-A 4-338718, as shown in
FIG. 7
, in order to planarize the upper side of the wiring
21
and the surface of the picture element electrode
22
, the picture element electrode is underlaid with a transparent insulating film
24
to level the region up, or the wiring is leveled down by etching in advance. Then, the level difference between the wiring
21
and the transparent electrode
22
of the picture element is planarized by covering the wiring
21
with an insulating film
25
. As the insulating film
25
, for example, a polyimide resin film is used.
In an active matrix-type liquid crystal display apparatus, higher resolution is required than before and, consequently, when a picture element pitch becomes narrower, improvements in an aperture ratio and transmittance are required to secure brightness.
In order to secure a desired aperture ratio, it is needed to enlarge the area of the picture element electrode as large as possible. However, since the wiring
21
or the wiring
21
covered with the insulating film
25
and the insulating film
24
do not make a consequential flat surface in the above conventional technique, a region where the picture element electrode
22
can be formed is limited only on the insulating film
24
. Therefore, based on the conventional constitutions shown in
FIGS. 6 and 7
, schematic cross-sectional views of constitutions in the case where the region in which the picture element electrode is to be formed is extended over not only the insulating film
24
but also the surface of the wiring
21
are shown in
FIGS. 8 and 9
.
In the constitution shown in
FIG. 8
, since the wiring
21
and the picture element electrode
22
directly contact with each other, the picture elements are always electrically connected with the wiring. In addition, in both constitutions shown in
FIGS. 8 and 9
, due to the influence by the difference in level of the wiring, it is impossible to make a flat picture element electrode. Consequently, liquid crystals can not be aligned uniformly in a rubbing step.
Thus, in the conventional constitution, since the picture element electrode
22
can not be formed in a region other than the region of the ins
Ogawa Yasuyuki
Sakai Tamotsu
Kennedy Jennifer M.
Niebling John F.
Nixon & Vanderhye P.C.
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